Filter-type digital diode phase shifter

ABSTRACT

Three PIN diodes form a T junction. When the series diodes are forward biased, the shunt diode is reverse-biased to effectively form a low pass circuit. When the series diodes are reversebiased, the shunt diodes are forward-biased to form effectively a high pass circuit. The cut off frequency of the low and high pass states are above and below, respectively, the operating band so that each T section selectively introduces a phase delay and advance in the low and high pass states, respectively.

United States Patent [191 [111 3,778,733

Rizzi Dec. 11, 1973 [5 FILTER-TYPE DIGITAL DIODE PHASE 3,356,865 12/1967Woster 333 31 R SHIFTER 3,453,564 7/1969 Russell 333/73 C Inventor:Peter A. Rizzi, Dedham, Mass.

Assignee: Alpha Industries, Inc., Woburn,

Mass.

Filed: May 8, 1972 Appl. No.2 251,116

US. Cl 333/31 R, 333/73 C, 333/97 S Int. Cl. I-I0lp 1/18 Field of Search333/31 R, 70 R, 73 R,

References Cited UNITED STATES PATENTS 12/1970 DiPiazza 333/31 R 8/1964Spallone 307/259 Primary ExaminerPaul L. Gensler Att0rneyCharles Hieken[5 7] ABSTRACT Three PIN diodes form a T junction. When the seriesdiodes are forward biased, the shunt diode is reversebiased toeffectively form a low pass circuit. When the series diodes arereverse-biased, the shunt diodes are forward-biased to form effectivelya high pass circuit. The cut off frequency of the low and high passstates are above and below, respectively, the operating band so thateach T section selectively introduces a phase delay and advance in thelow and high pass states, respectively.

1 Claim, 12 Drawing Figures PATENTEDBEBI 1 1911 I 5.718.133 MU 1 (If 2BIAS TO ALL 3 DIODES INJECTED HERE THRU AN RF CHOKE COIL.

A TIONAL INDUCTANCE REQUIRED) 5O OHM 5O OHM INPUT LINE OUTPUT'LINE PINF/G. DIODES sTATE A STATE B (SERIES DIODES- FORWARD BIASED) SERIESDIODES-REVERSE BIASED) SHUNT D|ODEs REVERSE BIASED SHUNT DIODES FORWARDBIASED XC i (OENJF/a 2A (LOAD) v (GEN.) F/G. 2B (LOAD) STATE A. STATE 8L E Q T o -/LOSS IN STATE 5 LOSS m STATE A LOSS OPERATING BAND v i eFREQUENCY PAIENTEDDEB! 1 ms FILTER-TYPE DIGITAL DIODE PHASE SHIFTERBACKGROUND OF THE INVENTION The present invention relates in general tophase shifting and more particularly concerns a novel microwave phaseshifter that advantageously employs PIN diodes to provide a reliabledigital phase shifter with relatively compact inexpensive reliableapparatus. The invention realizes large phase changes at microwavefrequencies in a very compact length.

It is an important object of the invention to provide an improved phaseshifter.

It is another object of the invention to achieve the preceding object atmicrowave frequencies.

It is a further object of the invention to achieve one or more of thepreceding objects while selectively effecting relatively large phasechanges at microwave frequencies in a compact length.

It is still a further object of the invention to achieve one or more ofthe preceding objects with apparatus that is mechanically andelectrically reliable and relatively easy and inexpensive to fabricate.

SUMMARY OF THE INVENTION According to the invention, there is at leastone filter section with means for selectively establishing low and highpass states with the cutoff frequencies in each state defining anoverlapping hand of operation in both states to produce a phasedifference in the overlapping band in the transmission characteristicsof the two states. Preferably each section comprises microwave diodeswhich, when not conducting, may be characterized by an equivalentcircuit having substantially an inductance in series with a capacitanceto form a series resonant circuit resonant at a frequency above theoverlapping or pass band of the system. Typically a section comprisesPIN diodes, two of which comprise series arms and one of which comprisesa shunt arm and means for biasing the series and shunt arms forconduction and nonconduction during mutually exclusive time intervals.

Numerous other features, objects, and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a combined diagrammatic andschematic circuit diagram of a section according to the invention;

FIGS. 2A and 2B are schematic circuit diagrams of equivalent circuits ofa section under states A and B, respectively, corresponding to low-passand high-pass states, respectively;

FIGS. 3A and 3B are schematic circuit diagrams of the equivalentcircuits of FIGS. 2A and 2B, respectively;

FIG. 4 is a graphical representation of insertion loss as a function offrequency to illustrate how the operating band is in the overlappingbands of low and high pass states;

FIG. 5 is a representation of the circuits of FIGS. 3A and 3B helpful inanalysis for transmission phase shift;

FIGS. 6A and 6B are the circuits of FIGS. 3A and 3B with normalizedvalues indicated for 180 phase shift;

FIG. 7 is a schematic circuit diagram of a section according to theinvention offering certain practical advantages; and

FIGS. 8A and 8B are schematic circuit diagrams of the equivalentcircuits of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to thedrawing, and more particularly FIG. 1 thereof, there is shown a combinedpictorial schematic circuit representation of an embodiment of theinvention. So as not to obscure the principles of the invention, detailsof the ground planes, terminals and the like are omitted, these aspectsof the structure being apparent to those skilled in the art. A typicalsection may include an input line 11 and an output line 12 characterizedby an impedance Z typically 50 ohms. Three diodes, such as PIN diodes,l3, l4 and 15 are connected poled as shown to a junction section 16which receives a biasing potential through r-f choke 17. The anode ofdiode 15 is connected to ground through line section 18. The cathodes ofdiodes 14, 15 and 16 may be connected in series with inductive sections21, 22 and 23, respectively, if additional series inductance is requiredfor a specific operating frequency range.

When series diodes l4 and 13 are forward-biased, shunt diode 15 isreverse-biased. Conversely, when diode 15 is forward-biased, diodes l4and 13 are reversed-biased.

The equivalent circuit of a diode, such as a PIN diode in a glasspackage, typically comprises at zero or reverse bias the seriescombination of lead inductance L, junction capacitance C,- andequivalent series resistance in the reverse-bias state R At forward biasthe lead inductance L is in series with an equivalent forward-biassereis resistance R, Neglecting losses, the equivalent circuit of asection in low-pass state A with the series diodes forward-biased andthe shunt diode reverse-biased is shown in FIG. 2A and in high-passstate B with the series diodes reverse-biased and the shunt diodeforward-biased is shown in FIG. 28. X is the inductive reactance of thediode lead inductance in series with an inductive section. The impedanceX is the capacitive reactance of a reverse-biased junction. If thecircuit is operated below the series resonance of an arm, X is less thanX and the circuits of FIGS. 2A and 23 may be represented by the circuitsof FIGS. 3A and 38, respectively. X =lX X As the bias state of thediodes' are switched, the circuit'changes from a low pass filter to ahigh pass filter. By establishing the parameter values so that thecutoff frequency of the low pass state is above the operating band andthat of the high pass state is below the operating band, the phaseshifter will exhibit low insertion loss in both states in the operatingband as indicated in FIG. 4. However, the low pass filter in state Aimparts a phase delay while the high pass filter in state B imparts aphase advance. The result is a low-loss transmission section in theoperating band having means for selectively shifting between phase delayand phase advance.

Referring to FIG. 5, there is shown a representation of the arms in thecircuit sections of FIGS. 2A and 2B helpful in analyzing the phase delayper section.

A I D 1 2 tan" [1 (x x p l/2 (x,

As an example, if Z, 50 X X 50 ohms (X, ohms), the circuit may berepresented on a normalized basis as having a vector impedance +j in theseries arms and -j in the shunt arm in state A as shown in FIG. 6A. Instate 8 the series arm impedances are each j and the shunt arm impedanceis +j as shown in FIG. 68. By inspection in this special case it can beseen that the phase delay in state A is 90 and 90 in state B to producea change of 180 between states A and B with one section. The physicalrealization in strip line is a unit less than an inch long at microwavefrequencies while in microstrip the length would be less than one-halfinch. Low values of phase shift may be obtained by changing the valuesof L and C so that x, is less than unity and x, is greater than unity inthe equation for phase delay above.

In an actual embodiment of the invention at a frequency of 2.0 GI-Iz,the loss in state A was only 1.60db and that in state B 3.8 db whileproviding a phase shift difference of 174 between the two states. Thatembodiment included 0.6 picofarad glass PIN diodes in the series armsand a 0.4 picofarad PIN diode in the shunt arm.

Referring to FIG. '7, there is shown a schematic circuit diagram of apreferred embodiment of the invention for even further reducinginsertion loss. This embodiment includes input and output terminals 31and 32 and a common or grounded line 33. Each series branch comprises adiode D1 shunted by a capacitor of value C1 in series with an effectiveinductance of value L1 between a respective one of input and outputterminals 32 and junction 34 to which a biasing potential is applied.The shunt network comprises a diode D2 in series with an inductance L2between junction 34 and grounded line 33, the latter series combinationbeing shunted by a capacitance of value C2. Diodes DI and D2 areextremely low capacitance diodes having a capacitance typically 0.05 to0.15 picofarads and function essentially only as switches. Capacitors C1and C2 are preferably high Q chip capacitors. The shunt arm comprises aparallel LC resonant circuit and is advantageous because it leads tomore practical element values. Principles of operation are essentiallythose described above in connection with the circuit in FIG. 2.

In low pass state A, diodes D1 are effectively short circuits whilediode D2 is an open circuit. In high pass state B diode D2 isessentially a short circuit while diodes D1 are open circuits. Specificvalues of L1 and CI, L2 and C2 are chosen so that the circuit thenapproximates a high pass T filter. With this circuit up to 180 of phasechange can be obtained with less than one db loss.

Referring to FIGS. 8A and 88, there are shown equivalent circuits of thecircuit of FIG. 7 in the A and B states respectively. In the A and 8states the series diodes are forward-biased and reverse-biased,respectively, and the shunt diode is reverse-biased and forward-biased,respectively. For determining phase shift, it is convenient to assumethat the resistances are negligible and may be neglected. Then thetransmission phase shift in the two biased states is given by thefollowing expression.

where x 01L, and b ==m (C C12) z in C for the A state;

x (11L,- l/mC and b wC, l/wL for the B state for the bias statesdescribed in FIG. 88. It is assumed that the junction capacitance of thediodes is so small that 1/wC,- -2i 12 2; and jI I- In other words:

A state. In the B state it is assumed that 011., l/wC,

l and wC 1/011 then x and b are negative so that the transmission phaseshift B +2X X 1)B tanll B Bl (L l/C) V (01 1 wh re and XB Z0 B8 YO If X,X,; 8,, 8,, l at midband, then the circuits of FIGS. 8A and 8Bcorrespond to the circuits of FIGS. 6A and 68 described above.

It is convenient to define impedance levels in the A and B statesas; V

Ki (2L /Cz) nd KB: Wis-J20 p tively, and cutoff fre uencies fA= (l/Qomen and fii= 112w) inn- 0,) where for frequencies which satisfy m(l/LiG) and m (l/LZCZ), L@,=L2/(l w L2C2) and C Ci/(lm L1C1). Usingthese definitions X (l/wC Zn) and BB: (Zn/(0&

In order to have a reasonably well-matched structure, li and K should beclose to Z,,. Choosing K V27 and K (Zti/" /2 Z0: (L /C 1)= L IC or(L1/C2)=(Le,, Ce,)=Z0 This results lnXA BA and X B,,. Similarly, areasonable compromise for setting the low and high pass cutoff is (ya/ftf lf where f, center frequency of the operating band. This resultsGiven Z,,,f and the required MD at midband, the following steps may befollowed to determine parameter values. 1. Find the midband value of X,(which is (DWBLJZO) for the required value of Ad from the followingexpression.

X [(3 X )/(1 XX)? 2 tan Ni /2.

The values of X, for common values of A4 are given in the followingtable. (Note that to a first approximation 2 tan AD/Z 3X 2. Determine Cfrom the equation.

3. Determine L and C from the equations.

L I/ZwD C and C1= 11201 1 It can be shown that values of A4 within 10percent of a predetermined nominal value may be achieved over a 20 to 30percent frequency range. However, greater uniformity of phase shift maybe achieved within a frequency range not exactly centered about thenominal center frequency f,,. It may be advantageous to plot DA DB as afunction of frequency experimentally or through computer simulationtechniques in connection with selecting an optimum frequency range for apredetermined phase shift.

It can be shown that good VSWR in both the A and B states can beobtained over a relatively wide bandwidth for a phase difference up to90". For a phase difference of 180, the VSWR is good over a bandwidth ofto percent of center frequency.

lt can also be shown that the insertion loss is roughly constant over apercent bandwidth and less than a decibel for a 90 phase shift, and evensmaller for lesser phase shifts.

It can also be shown that a unit operating at C band can readily handle10 watts of incident r-f power. For Z 50 ohms and 200 volt PIN diodes,the maximum allowable r-f power before appreciable nonlinearity occursis about 800 watts peak. For 400 volt diodes, the allowable peak poweris roughly 3 kilowatts.

The invention is thus capable of wide band low-loss performance in anexceptionally compact structure especially suitable for microstripfabrication.

There has been described a novel digital phase shifter characterized byrelatively low insertion loss, relatively large available phase shift inan exceptionally compact physical structure while operating reliably andbeing relatively inexpensive to fabricate. It is evident that thoseskilled in the art may now make numerous modifications and uses of anddepartures from the specific embodiments described herein withoutdeparting from the inventive concepts. Consequently, the invention is tobe construed as embracing each and every novel feature and novelcombination of features present in or possessed by the apparatus andtechniques herein disclosed and limited solely by the spirit and scopeof the appended claims.

What is claimed is:

1. Microwave phase shifting apparatus comprising,

means defining an input and output,

means including inductive and capacitive elements intercoupling saidinput and output,

means including nonlinear elements for selectively shifting the networkformed by said elements between a low pass state and a high pass state,

the parameter values of said elements coacting to establish the cutofffrequency in said high pass state below the cutoff frequency in said lowpass state to define therebetween an operating band in which the phaseshift imparted between said input and said output is different in saidlow pass state than in said high pass state,

said nonlinear elements being unilaterally conducting devices and saidmeans for selectively shifting comprising means for selectivelyrendering said unilaterally conducting devices conducting andnonconducting,

there being first and second ones of said unilaterally conductingdevices in series arms between said input and said output and a third ofsaid unilaterally conducting devices connected between the junction ofsaid first and second devices and a common line,

and means for establishing said first state by rendering said first andsecond devices conductive and said third device nonconductive and forestablishing said second state by rendering said first and secspectiveinductive element.

1. Microwave phase shifting apparatus comprising, means defining aninput and output, means including inductive and capacitive elementsintercoupling said input and output, means including nonlinear elementsfor selectively shifting the network formed by said elements between alow pass state and a high pass state, the parameter values of saidelements coacting to establish the cutoff frequency in said high passstate below the cutoff frequency in said low pass state to definetherebetween an operating band in which the phase shift imparted betweensaid input and said output is different in said low pass state than insaid high pass state, said nonlinear elements being unilaterallyconducting devices and said means for selectively shifting comprisingmeans for selectively rendering said unilaterally conducting devicesconducting and nonconducting, there being first and second ones of saidunilaterally conducting devices in series arms between said input andsaid output and a third of said unilaterally conducting devicesconnected between the junction of said first and second devices and acommon line, and means for establishing said first state by renderingsaid first and second devices conductive and said third devicenonconductive and for establishing said second state by rendering saidfirst and second devices nonconductive and said third device conductive,said third device being in series with an inductive element and thelatter series combination being shunted by a capacative element to forma shunt parallel resonant circuit, each of said first and second devicesshunting a respective capacitive element and in series with a respectiveinductive element.